Recently, as computer networks have grown and expanded, it has become apparent that there exists a need to be able to connect local area networks (LAN) and wide area networks (WAN) together in order to build larger, extended local area and wide area networks. In order to interconnect individual local area and wide area networks, devices known as bridges have been developed. In practice, when connecting two separate networks together, the bridge is a device which coexists on the two networks and thereby acts as a gateway for the transmission of data from one network to the other. However, rather than passing all data packets which are presented to the bridge, from one network to the other, it is well known that in order to increase bridge throughput, and not to allow a bridge to place an undue burden on the networks to which it is attached, the bridge must prevent (a) data packets with local destinations from crossing the bridge, and (b) data packets from crossing one network to another when such crossing will not put the packet on a path that will allow it to successfully reach its destination.
When networks are small and localized, individual two port bridges for connecting together two networks are sufficient to carry data and still provide acceptable response times. However, as data networks grow and interconnect to more local area and wide area networks, the limitations found in bridges of the prior art, as well as those limitations inherent within the network specifications themselves, make the economical expansion of networks and efficient transmission of data more difficult. This is especially true in the case of networks which utilize source routing, the most popular example of which is the IBM Token Ring Network.
Under the source routing protocol supported by the IBM Token Ring Network Architecture, a data packet which is to be relayed from one network to another network, across a bridge, includes source routing information which defines the exact route the data packet should traverse. However, because of limitations in the present token ring source routing protocol, currently available source routing token ring bridges are able to support only a single pair of input and output connections (such as a LAN to LAN connection or an LAN to WAN connection). Unfortunately, since these prior art bridges may only pass data from one pre-defined network to another pre-defined network, if a packet's destination is located some distance away from the source, (i.e., the destination is at a remote location electrically connected through at least one additional bridge on another network), then, depending upon the network topography, the packet may have to pass through many bridges and rings (up to seven bridges and a total of eight rings) before arriving at its destination, thus placing an unacceptable load on the interconnected networks and thereby slowing system throughput.
One example of a local area network bridge is illustrated in U.S. Pat. No. 4,922,503, issued to Leone on May 1, 1990. Leone illustrates a bridge for interconnecting local area networks. In Leone, the bridge makes a decision on whether or not to forward a data packet across the bridge based upon the packet's destination address. Unfortunately, this technique of "destination address filtering" is not helpful in constructing a multi-port bridge (adapted to be connected to more than two networks) to be used in a token ring environment, since as noted, the token ring architecture utilizes a "source routing" protocol, in which the originating station determines the route a packet should take to its destination.
Therefore, noting the problems described above, and understanding the unavailability of a solution using destination address filtering, one example of an improved bridge for use with token ring source routing is noted in U.S. Pat. No. 4,901,312 "Remote Interconnection of Local Area Networks" issued to Hui et al. on Feb. 13, 1990. Under the Hui patent, token ring type local area networks may be connected together by the use of virtual or pseudo rings, which provide a technical fiction used to create a pseudo "network" between remote token rings. In Hui, each actual token ring network is connected to a pseudo token ring network by a respective half-bridge. The half-bridge then enters a number representing the pseudo ring in the source routing information field of the packet it forwards over the pseudo ring network to a remote half-bridge and remote ring. By using the disclosure of Hui, wide area networks are considered token ring local area networks by the respective half-bridges for the purpose of specifying source routing information. This permits a half-bridge comprised of a LAN module and a WAN module to be connected to another remotely located WAN or LAN module half-bridge. However, by using the architecture shown in Hui, LAN segment numbers must be assigned to wide area network links, which diminishes the already limited number of LAN segments that may be linked together. In particular, assigning LAN segment numbers to WAN links essentially halves the number of real LAN's that can be connected. As noted in the original IEEE 802.5 source routing specification, up to 8 segment number fields may be provided in a packet, which, in turn, allows the packet to specify a destination up to seven segments away. However, if each of the segments are connected by bridges using the Hui et al. method, a packet is limited to finding a destination which is a maximum of only three segments away, since the remaining four segments are used up by being assigned to intermediate "pseudo rings".
Recognizing this limitation, the IEEE has recently received proposals to change the current token ring standard by lengthening the information fields provided within a data packet in order to accommodate more segment number fields and longer ring-in/bridge number/ring-out fields. This additional length will allow "virtual" or "pseudo" rings to be constructed out of existing LAN links, in a manner similar to that taught in Hui, without severely limiting the maximum number of "hops" a destination may be away from a source. However, if the changes proposed by the IEEE are implemented, there will be a class of equipment already on the market which will be non-compliant with the revised specification.
Accordingly, it has been determined that the need exists for an improved multiport source routing bridge apparatus which may operate in accordance with the IBM Token Ring Protocol or IEEE 802.5 Protocol to interconnect several ports such as a LAN to LAN to LAN or LAN to WAN to WAN and which may also be used for connecting multiple virtual circuits, as may be found in a switched packet network over a single WAN port, without limiting the "fan-out" of the bridge.